Waterjet systems (e.g., abrasive jet systems) are used in precision cutting, shaping, carving, reaming, and other material-processing applications. During operation, waterjet systems typically direct a high-velocity jet of fluid (e.g., water) toward a workpiece to rapidly erode portions of the workpiece. Abrasive material is typically added to the fluid to increase the rate of erosion. When compared to other material-processing systems (e.g., grinding systems, plasma-cutting systems, etc.) waterjet systems can have significant advantages. For example, waterjet systems often produce relatively fine and clean cuts, typically without heat-affected zones around the cuts. Waterjet systems also tend to be highly versatile with respect to the material type of the workpiece. The range of materials that can be processed using waterjet systems includes very soft materials (e.g., rubber, foam, leather, and paper) as well as very hard materials (e.g., stone, ceramic, and hardened metal). Furthermore, in many cases, waterjet systems are capable of executing demanding material-processing operations while generating little or no dust, smoke, and/or other potentially toxic byproducts.
In a typical waterjet system, a pump pressurizes fluid to a high pressure (e.g., 40,000 psi to 100,000 psi or more). Some of this pressurized fluid is routed through a cutting head that includes an orifice element having an orifice. The orifice element can be a hard jewel (e.g., a synthetic sapphire, ruby, or diamond) held in a suitable mount (e.g., a metal plate). Passing through the orifice converts static pressure of the fluid into kinetic energy, which causes the fluid to exit the cutting head as a jet at high velocity (e.g., up to 2,500 feet-per-second or more) and impact a workpiece. After eroding through a portion of a workpiece, the jet typically is dispersed in a pool of fluid held within a catcher (e.g., a catcher tank) positioned below the workpiece, thereby causing the kinetic energy of the jet to dissipate. A jig including spaced apart slats can be used to support the workpiece over the catcher safely and non-destructively. The jig, the cutting head, the workpiece, or a combination thereof can be movable under computer and/or robotic control such that complex processing instructions can be executed automatically.
Certain materials, such as composite materials, brittle materials, certain aluminum alloys, and laminated shim stock, among others, may be difficult to process using conventional waterjet systems. For example, when a jet is directed toward a workpiece, the jet may initially form a cavity in the workpiece and hydrostatic and/or stagnation pressure from fluid within the jet may act on sidewalls of the cavity. This can cause weaker parts of composite materials to preferentially erode. In the case of layered composite materials, for example, hydrostatic and/or stagnation pressure from a jet may erode binders between layers within the workpiece and thereby cause the layers to separate. Similarly, in the case of fiber-containing composite materials, hydrostatic and/or stagnation pressure from a jet may exceed the bond strength between the fibers and the surrounding matrix, which can also cause delamination. As another example, when a jet is directed toward a workpiece made of a brittle material (e.g., glass), the load on the workpiece during piercing may cause the workpiece to spall and/or crack. Similarly, spalling, cracking, or other damage can occur when jets are used to form particularly delicate structures in both brittle and non-brittle materials. Other properties of jets may be similarly problematic with respect to certain materials and/or operations.
One conventional technique for mitigating collateral damage to a workpiece (e.g., a workpiece made of a composite and/or brittle material) includes piercing the workpiece with a jet formed at a relatively low pressure and then either maintaining the low pressure during the remainder of the processing or ramping the pressure upward after piercing the workpiece. At relatively low pressures, waterjet processing is often too slow to be an economically viable option for large-scale manufacturing. Furthermore, conventional techniques for ramping pressures upward can also be slow and typically decrease the operational life of at least some components of conventional waterjet systems. For example, at least some conventional techniques for ramping pressure upward include controlling a pump and/or a relief valve of a waterjet system to increase the pressure of all or substantially all of the pressurized fluid within the waterjet system. This causes a variety of components of the waterjet system (e.g., valves, seals, conduits, etc.) to be repeatedly exposed to the fluid at both low and high pressures. Over time, this pressure cycling can lead to fatigue-related structural damage to the components, which can cause the components to fail prematurely. Greater numbers of pressure cycles and greater pressure ranges within each cycle can exacerbate these negative effects. The costs associated with such wear (e.g., frequent part replacements, other types of maintenance, and system downtime) tend to make such approaches impractical for most applications. For example, in material-processing applications that involve repeatedly cycling a jet between piercing and cutting operations and/or starting and stopping a jet (e.g., to form spaced-apart openings in a workpiece made of a composite or brittle material), the associated cycling of fluid pressure can cause unacceptable wear to conventional waterjet systems and make use of such systems for these applications cost prohibitive.